Cracks that form within materials can be difficult to detect and almost impossible to repair. A successful method of autonomically repairing cracks that has the potential for significantly increasing the longevity of materials has been described, for example, in U.S. Pat. No. 6,518,330. This self-healing system includes a material containing, for example, solid particles of Grubbs catalyst and capsules containing liquid dicyclopentadiene (DCPD) embedded in an epoxy matrix. When a crack propagates through the material, it ruptures the microcapsules and releases DCPD into the crack plane. The DCPD then mixes with the Grubbs catalyst, undergoes Ring Opening Metathesis Polymerization (ROMP), and cures to provide structural continuity where the crack had been.
A challenge in designing autonomically self-healing composite materials such as these is to isolate the polymerizer (i.e. DCPD) from the corresponding activator (i.e. Grubbs activator) in the polymer matrix, while still providing for sufficient contact between the polymerizer and activator when a crack is formed in the matrix. In particular, it can be challenging to ensure that the activator is protected during the formation and use of the composite, and that it is sufficiently distributed within the polymer matrix so as to be available to form a polymer from the polymerizer. In conventional self-healing composite systems, the activator has been dispersed as solid particles of pure activator, or as particles containing the activator and an encapsulant such as wax. The use of capsules containing an activator has been described, for example, in Cho et al. Adv. Mater. 2006, 18, 997. In this system, a polymerizer was present as phase-separated droplets in the polymer matrix, rather than in capsules.
Composite materials that contain a polymerizer without an added activator have not shown autonomic self-healing behavior. In the DCPD systems described above, when control experiments were performed in the absence of catalyst, no healing was observed (White, S. R. et al. Nature 2001, 409, 794.). In another system, healing of polyester composites was investigated, based on polymerization of a monomer together with activating groups bound to the polyester matrix (D. Jung, et al., vol. MD-80, in Proceedings of the ASME International Mechanical Engineering Conference and Exposition, 265-275, 1997). The activating groups investigated in this system included carboxylic acid groups, ester groups, unsaturated groups, epoxide groups and amine groups. None of these native activating groups was able to provide self-healing of the matrix when contacted with the monomer from the capsules.
Another approach to healing cracks in a material does not use polymerizers or activators at all. Rather, this approach involves heating the material to bond the crack faces. The heating may be combined with the addition of a solvent to facilitate physical and/or chemical bonding between the polymer chains on either side of the crack face (Shen, J.-S. et al. J. Mater. Res. 2002, 17, 1335; Lin, C. B. et al. Poly. Eng. & Sci. 1990, 30, 1399; Hsieh, H.-C. Polymer 2001, 42, 1227). A crack in an epoxy polymer without any additives has been reported to heal at elevated temperatures, particularly when the polymer was formed with non-stoichiometric amounts of the epoxide and amine starting materials (Rahmathullah, A. M. et al. “Healing Behavior of DGEBA Epoxy Cured with a Cycloaliphatic Diamine.” Proceedings of the First International Conference on Self-Healing Materials, Apr. 18-20, 2007, Noordwijk, The Netherlands, abstract). The drawback to this approach is that the material cannot self-heal autonomously, as the material must be heated by an outside source.
It is desirable to provide a self-healing material that includes fewer components than conventional self-healing materials, yet can autonomically self-heal when a crack occurs. It is also desirable to provide self-healing materials that do not include components that are expensive, unstable and/or difficult to process.